1. Field of the Invention
The present invention relates to a transmitter circuit including a power amplifier using a gallium arsenide FET (Field Effect Transistor) which is used in, for example, a mobile telecommunication system but the present invention is not restricted to the system.
2. Description of the Prior Art
A conventional transmitter circuit used in, for example, a mobile telecommunication system comprises a power amplifier having an FET as an amplifying element. Recently, a gallium arsenide FET has been used as the amplifying element because the gallium arsenide FET is more efficient than silicon transistors. The power amplifier is for power amplifying a signal delivered thereto to send out a power amplified signal to the transmission line as the transmission signal. A biassing voltage generator is coupled to the gallium arsenide FET and is responsive to an input power supplied thereto to produce a biassing voltage of a negative level determined by a voltage level of the input power. The biassing voltage generator delivers the biassing voltage to the gallium arsenide FET as a gate biassing voltage for the gallium arsenide FET. A controller is used for producing, upon start of the transmitter circuit, a first power-on signal and thereafter a second power-on signal with a time delay from producing the first power-on signal. A first power supply is coupled to the control means and the biassing voltage generating means. The first power supply generates a first electric power in response to the first power-on signal to deliver the first electric power to the biassing voltage generator as the input power. A second power supply is coupled to the control means and the power amplifier. The second power supply produces, in response to the second power-on signal, a second electric power of a voltage level higher than the first electric power to supply the second electric power to the power amplifier.
At a start of the transmitter circuit, the gallium arsenide FET is at first supplied with the gate biassing voltage from the biassing voltage generator but the power amplifier is not yet supplied with the second electric power. This is necessary to protect the gallium arsenide FET from damage. This is because when the gate biassing voltage is not supplied to the FET, application of the voltage across the drain and the source makes a large current flow therethrough and results in damage of the FET.
When an undesired signal is applied to the power amplifier under the start condition, the undesired signal is output from the power amplifier and is transmitted to the transmission line. The conventional transmitter circuit usually comprises a multistage amplifier for previous signal amplification prior to the power amplifier. The multistage amplifier is also energized by an electric power produced in response to the second power-on signal. In order to avoid transmission of the undesired signal, the stage number of the multistage amplifier is increased so as to attenuate the undesired signal at the multistage amplifier. This increases parts and electric power consumption.